The use of different air quality markers (metrics) for surface ozone calculated from the same time series can result in different trend patterns. This outcome is important to researchers, as well as policymakers and regulators, who use exposure metrics to assess how changes in ozone levels affect human health, vegetation, and climate. That's one conclusion from a new metrics assessment based on the Tropospheric Ozone Assessment Report -or TOAR-, an effort by the International Global Atmospheric Chemistry Project to create the world’s largest database of surface ozone observations from all available ozone monitoring stations around the globe. The paper was published on Friday 6 April in the journal Elementa: Science of the Anthropocene.

The paper is available at the Elementa website at: https://www.elementascience.org/article/10.1525/elementa.279/ . 24 international researchers worked on it: among them, Elena Paoletti (Cnr-Ipsp), who has been a member of the TOAR Steering Committee since the inception of the program in 2014, anticipates that TOAR will provide scientists, regulators, and policymakers with better insight about spatial and temporal variation that relate to climate change, human health, and crop/ecosystem around the world.

“Key components of the Tropospheric Ozone Assessment Report (TOAR) are the use of metrics that are biologically defensible, as well as the use of statistical methods that adhere to stringent scientific principles,” said Allen Lefohn, the lead author of the paper. “With the TOAR database and its open data web services it has become possible to evaluate the different metrics that are used throughout the world in a globally consistent manner, and thus compare ozone impacts in different countries on a much more objective basis,” adds Martin Schultz, who assembled and curated the TOAR database.

Tropospheric or ground-level ozone is a greenhouse gas and air pollutant that, at elevated levels, is detrimental to human health and crop and ecosystem productivity. Ground-level ozone is a secondary pollutant, meaning that it is not emitted directly, but forms when sunlight triggers reactions between natural and human-caused chemical emissions, known as ozone precursor gases. Emissions from vehicles, power plants, industrial operations, and other human activities are a primary cause of surface ozone, which is one of main pollutants regulated in the European Union.

Scientists have understood that precursor gases that form ozone have been declining in North America and Europe since the 1990s, but levels have been increasing in Asia. However, limited and scattered ozone datasets left scientists unable to answer basic questions about the distribution and trends in ozone pollution in many parts of the world: In which regions of the world do people face the greatest ozone exposure? To what extent is ozone increasing in developing countries? Have air quality regulations reduced ozone levels in developed nations?

To address those and other questions involving the TOAR effort, it was necessary to identify and describe the various metrics used in the TOAR program. The metrics paper has brought focus on explaining why it is important to (1) use biologically defensible metrics and (2) apply statistical methods that adhere to stringent scientific principles. An important outcome of the effort was the recognition that the use of different metrics calculated from the same time series can result in different trend patterns. This outcome is highly relevant to researchers, as well as policymakers and regulators, who use exposure metrics to assess how changes in ozone levels affect human health and vegetation, as well as the efficiency of air pollution control strategies.

The paper provides the following:

- adescription of 25 metrics, which are used for assessing spatial and temporal trends by environmental agencies and researchers around the world (4 for model-measurement comparison, 5 for characterization of ozone in the free troposphere, 11 for human health impacts, and 5 for vegetation impacts).

- the scientific rationale for the selection of each of the 25 metrics.

- a detailed description of the statistical methods based on stringent scientific principles used in the TOAR program.

- a comparison of the trend behavior for each of the ozone impact metrics when using the same surface ozone concentration time series.

Twenty-four institutions worldwide were involved in the research effort summarized in the paper. The research groups were: Stockholm Environment Institute in the UK, NERC Centre for Ecology & Hydrology in the UK, University of Edinburgh in the UK, A.S.L. & Associates in the U.S., Alion Science and Technology, Inc. in the U.S., U.S. Environmental Protection Agency, University of North Carolina-Chapel Hill in the U.S., NOAA Geophysical Fluid Dynamics Laboratory in the U.S., CNR National Research Council of Italy, Forschungszentrum Jülich in Germany, Institute for Sustainable Plant Protection in Italy, Italian National Agency for New Technologies in Italy, Institute of Atmospheric Composition in Chinese Academy of Meteorological Sciences in Beijing China, The Hong Kong Polytechnic University in Hong Kong, China, Appalachian State University in the US, US Department of Agriculture Forest Service in the U.S., Air Quality Research Division in Canada, University of British Columbia in Canada, Research Center for Eco-Environmental Sciences in the Chinese Academy of Sciences in Beijing China, Institute of Soil Sciences in the Chinese Academy of Sciences in Nanjing China, The University of Tokyo in Japan, ACRI-HE in France, Norwegian Institute for Air Research (NILU) in Norway, and Università Cattolica del Sacro Cuore in Italy.

The TOAR database of surface ozone metrics is publicly available and can be used by scientists and policymakers around the world to quantify the impacts of ozone on human health and vegetation.

TOAR is a project of the International Global Atmospheric Chemistry project, with support from NOAA, Forschungszentrum Jülich, and the World Meteorological Organization.